1. Field of the Invention
Apparatus for controlling the output voltage of a generator by using semiconductors have been used. Recently, a need for safety in these apparatus has increased and it has become an important problem to utilize a safety circuit in compact form. The present invention fills this need.
2. Description of the Prior Art
Referring to FIG. 1, the conventional apparatus will be described.
In FIG. 1, the reference 1 designates an AC generator including an armature coil 2 and a field coil 3. The armature coil 2 is connected through a rectifier 4 to a battery 5.
One end of the field coil 3 is connected through a switch 6 to the battery 5. The other end of the field coil 3 is connected to a collector of a NPN type power transistor 9 which functions as a voltage adjustment device 8 in a voltage control apparatus 7 and is connected through the emitter to ground.
The base of NPN type power transistor 9 is connected through a resistor 10 and the key switch 6 to the battery 5. The collector-emitter of an NPN type transistor 11 is connected in parallel to the base-emitter of the power transistor 9.
The base of the NPN type transistor 11 is connected through a Zener diode 14 to the middle point of the series connection of a resistor 12 and a resistor 13 for detecting the output voltage after recification of the AC generator 1. The reference 15 designates a diode connected in parallel to the field coil 3. When the NPN type power transistor 9 is in the off state, the diode 15 absorbs the voltage induced in the field coil 3.
One end of a charge display lamp 16 is connected through the key switch 6 to the battery 5 and the other end is connected to the collector of a NPN type transistor 18 which functions as a discharge display circuit 17 of the voltage control apparatus 7 and is connected through its emitter to ground. The base of the NPN type transistor 18 is connected through a resistor 19 and the key switch 6 to the battery 5 and is also connected to the collector and emitter of an NPN type transistor 20 disposed in parallel to the base and emitter of the transistor 18.
The base of the NPN type transistor 20 is connected through a resistor 21 and a diode 22 to the neutral point of the AC generator 1. A capacitor 23 is connected to smooth the waveform between the contact of the resistor 21 and the diode 22 and ground.
In this voltage control apparatus, when the key switch 6 is turned on at the initiation of the driving, the base current of the NPN type power transistor 9 of the voltage adjustment device 8 is passed from the battery 5 through the key switch 6 and the resistor 10. At the same time, the exciting current is passed from the key switch 6 through the field coil 3 and the collector and emitter of the NPN type power transistor 9.
On the other hand, in charge display circuit 17, the base current of the NPN type transistor 18 is passed through the battery 5, the key switch 6 and the resistor 19. The current is passed from the key switch 6 through the charge display lamp 16 and the collector and emitter of the NPN transistor 18 so that the charge display lamp 16 is ignited to indicate the non-generation by AC generator 1.
When the speed of rotation of the AC generator 1 increases to generate a voltage to the armature coil 2 and to initiate the charge through the rectifier 4 to the battery 5, the base current of the NPN type transistor 20 is passed from the neutral point of the AC generator 1 through the diode 22 and the resistor 21 whereby the collector-emitter of the NPN type transistor 20 is turned on to form a short circuit between the base and emitter of the NPN type transistor 18. The base current of the NPN type transistor 18 is not passed to extinguish the charge display lamp 16 to indicate the generation by AC generator 1.
When the output voltage of the AC generator 1 becomes higher than a predetermined value, as is known, the current is passed through the Zener diode 14 and the base and emitter of the NPN type transistor 11 by the voltage shunt of the resistor 12 and the resistor 13 to detect the voltage at voltage adjustment device 8 and a short-circuit is formed between the base and emitter of NPN type power transistor 9. The current of the field coil is interrupted in the off state of NPN type power transistor 9. Accordingly, the output voltage of the generator 1 is decreased causing the off state of the NPN type transistor 11 and the on state of the NPN type power transistor 9.
This operation is repeated to maintain the output voltage constant. In this apparatus, when the voltage control function is lost because of the short-circuit of the collector and emitter of the NPN type power transistor 9 of the voltage adjustment device 8, or the open-circuit of the NPN type transistor 11 or the Zener diode 14, or a disconnection of the resistor 12, the battery 5 is charged by the AC generator 1 under abnormal overvoltage. Over current is passed to cause a rise in temperature of the battery 5 and the wiring, and as a result thereof a dangerous accident may occur. Also, there is a disadvantage of interruption of an electric load (not shown) connected in paralled to the battery 5 caused by the overvoltage.